In interpreting these lines, it's important to bear in mind exactly what the Fourth Tempter is trying to do. All the Tempters are showing Thomas a false path -- a path that will lead to damnation. Their temptations grow progressively more subtle, until the Fourth Tempter arrives. Remember that when the Fourth Tempter first appears, Thomas is surprised:
Who are you? I expected
Three visitors, not four.
The Fourth Tempter thus represents a temptation that comes from very deep inside Thomas's own mind, deep enough that he has never confronted the possibility the Tempter represents. The Tempter points out that Thomas has considered the glory of martyrdom before ("You have thought of that too"). But he hasn't allowed the thought to become conscious. Perhaps for a long time it wasn't relevant, but now that it's becoming a real possibility, he's actively resisting it. Once he has the thought, it becomes possible for him to seek martyrdom for selfish reasons -- for the sake of lasting personal glory. To be martyred because the alternative was to betray God is one thing; to be martyred because you like the idea of being a saint... Well, as Thomas puts it himself:
The last temptation is the greatest treason;
To do the right deed for the wrong reason.
It's vital to look at the immediate context, too. Thomas has just said to the Tempter:
Can I neither act nor suffer
The previous tempters wanted Thomas to do something: to act. The Fourth Tempter wants Thomas to endure something: to suffer. Thomas is dismayed, because he had thought that by not taking action he could avoid damnation. But in this speech, the Fourth Tempter points out that there isn't really a definitive difference between acting and suffering. The actor (the one who acts) must also be a patient (one who suffers). And the same applies in reverse. In effect, the Fourth Tempter is trying to convince Thomas that there is no way to avoid responsibility -- quite literally, that Thomas is damned if he does and damned if he doesn't. And if it is, in fact, impossible to avoid damnation, Thomas might as well go down in a blaze of glory, and seek martyrdom for the sake of later sanctification.
But finally, it's important to recognise that Eliot isn't trying to present the Fourth Tempter as being fundamentally right. All the Tempters are to some degree reasonable, all of them have something genuinely tempting to offer. But their view of the world is skewed and incomplete, and Thomas ultimately manages to resist all of them. His final speech in Part 1 includes the line
I shall no longer act or suffer, to the sword's end.
In other words, the Tempter may be right to suggest there's no bright line separating suffering from acting, but he's also wrong to imply that that leaves Thomas with no way out. I don't fully understand what Eliot means when he has Thomas say he will "no longer act or suffer" -- it's not as if Thomas dies or goes into a coma at that moment -- and I would tentatively suggest that what Eliot has in mind is something like perfect submission to God's will and complete acceptance of whatever comes next. In his sermon, later in the play, he says "A martyrdom is always the design of God." This suggests, to me at least, that Thomas can see his martyrdom coming and is accepting of it -- but is also willing to let it pass by, if it's not God's will.